Refrigerant vapor compression systems are well known in the art and commonly used for conditioning air to be supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression systems are also commonly used in transport refrigeration systems for refrigerating air supplied to a temperature controlled cargo space of a truck, trailer, container or the like for transporting perishable items. Traditionally, most of these refrigerant vapor compression systems operate at subcritical refrigerant pressures and typically include a compressor, a condenser, and an evaporator, and expansion device, commonly an expansion valve, disposed upstream, with respect to refrigerant flow, of the evaporator and downstream of the condenser. These basic refrigerant system components are interconnected by refrigerant lines in a closed refrigerant circuit, arranged in accord with known refrigerant vapor compression cycles, and operated in the subcritical pressure range for the particular refrigerant in use. Refrigerant vapor compression systems operating in the subcritical range are commonly charged with fluorocarbon refrigerants such as, but not limited to, hydrochlorofluorocarbons (HCFCs), such as R22, and more commonly hydrofluorocarbons (HFCs), such as R134a, R410A and R407C.
In today's market, greater interest is being shown in “natural” refrigerants, such as carbon dioxide, for use in air conditioning and transport refrigeration systems instead of HFC refrigerants. However, because carbon dioxide has a low critical temperature, most refrigerant vapor compression systems charged with carbon dioxide as the refrigerant are designed for operation in the transcritical pressure regime. In refrigerant vapor compression systems operating in a subcritical cycle, both the condenser and the evaporator heat exchangers operate at refrigerant temperatures and pressures below the refrigerant's critical point. However, in refrigerant vapor compression systems operating in a transcritical cycle, the heat rejection heat exchanger, which is a gas cooler rather than a condenser, operates at a refrigerant temperature and pressure in excess of the refrigerant's critical point, while the evaporator operates at a refrigerant temperature and pressure in the subcritical range.
Control of refrigerant charge in a subcritical refrigerant vapor compression system is relatively simple. Conventional subcritical refrigerant vapor compression systems may also include a receiver disposed in the refrigerant circuit downstream of the condenser and upstream of the expansion device. Liquid refrigerant from the condenser enters the receiver tank and settles to the bottom of the tank. As this liquid will be at saturated temperature, refrigerant vapor will fill the space in the tank not filled by liquid refrigerant. Liquid refrigerant is metered out of the receiver tank by the expansion valve which controls refrigerant flow to the evaporator. As the operating conditions of the subcritical refrigerant vapor compression system change, the charge requirements for the system will change and the liquid level in the receiver tank will rise or fall, as appropriate, to establish a new equilibrium liquid level.
If at any point in operation there is too much refrigerant charge circulating in the system, the rate of liquid refrigerant entering the receiver tank will exceed the rate of refrigerant leaving the receiver tank and the liquid level within the receiver tank will rise until equilibrium is reached between the rate of liquid entering the receiver tank and the rate of liquid leaving the receiver tank with the excess liquid remaining stored in the receiver tank. If an any point in operation there is too little refrigerant charge circulating in the system, the rate of liquid refrigerant entering the receiver tank will be less than the rate of liquid leaving the receiver tank and the liquid level within the receiver tank will drop as liquid returns from the receiver tank to the refrigerant circuit to circulate therethrough. The liquid level within the receiver tank will continue to drop until a new equilibrium is established between the rate of liquid entering the receiver tank and the rate of liquid leaving the receiver tank.
In a transcritical refrigerant vapor compression system, however, controlling the system refrigerant charge is more complex because the compressor high side refrigerant leaving the gas cooler is above the refrigerant's critical point and there is no distinct liquid or vapor phase and thus the charge present in the receiver becomes a function of temperature and pressure which may not respond in a desirable manner to system charge requirements. One system commonly proposed for use in connection with charge regulation on transcritical refrigerant vapor compression systems includes a flash tank disposed downstream of the gas cooler and upstream of the expansion device with respect to refrigerant flow. A flow regulating throttling valve is disposed in the refrigerant line at the entry to the flash tank. Supercritical pressure refrigerant gas passing through the flow regulating throttling valve drops in pressure to a subcritical pressure forming a subcritical pressure liquid/vapor refrigerant mixture which collects in the flash tank with the liquid refrigerant settling to the lower portion of the tank and the vapor refrigerant collecting in the portion of the flash tank above the liquid refrigerant. A float valve is provided within the flash tank and operatively connected by a mechanical linkage mechanism to control operation of the flow regulating throttling valve to maintain a predetermined liquid level within the flash tank. If the liquid level in the flash tank should raise, the float raises therewith and causes the throttle valve to close further to restrict the flow of refrigerant into the flash tank. Conversely, if the liquid level in the flash tank should drop, the float drops therewith and causes the throttle valve to open more to increase the flow of refrigerant into the flash tank. The liquid level with the flash tank is thus maintained at the predetermined liquid level which is selected to ensure that only liquid phase refrigerant returns to the refrigerant circuit from the lower region of the flash tank to pass through the expansion device upstream of the evaporator and that only vapor phase refrigerant returns to the refrigerant circuit from the upper region of the flash tank to be passed back to the compressor for recompression through an economizer line.
U.S. Pat. No. 5,174,123 discloses a subcritical refrigerant vapor compression system including a compressor, a condenser, and an evaporator, with a float-less flash tank disposed between the compressor and the evaporator. Refrigerant flows into the flash tank from the condenser at saturated conditions. The flow of refrigerant into the flash tank is controlled by selectively opening or closing a sub-cooling valve to maintain a desired degree of sub-cooling. The flow of liquid refrigerant out of the flash tank to the evaporator is controlled by a suction superheat thermostatic expansion valve. Refrigerant vapor collecting in the flash tank above the liquid refrigerant therein is returned to the compressor, being injected into an intermediate pressure stage of the compressor. Because of the float-less nature of the flash tank, the disclosed refrigerant vapor compression system is said to be particularly suited for transport refrigeration applications.
U.S. Pat. No. 6,385,980 discloses a transcritical refrigerant vapor compression system including a float-less flash tank disposed between a gas cooler and an evaporator and a controller regulating valves in response to the sensed refrigerant pressure in the gas cooler to control the amount of charge in the flash tank to regulate the refrigerant pressure in the gas cooler. The controller controls the flow of supercritical refrigerant from the gas cooler into the flash tank by regulating an in-line expansion valve on the entry side of the flash tank and the flow of liquid refrigerant from the flash tank to the evaporator by regulating an in-line expansion valve on the exit side of the flash tank. Refrigerant vapor collecting in the flash tank above the refrigerant liquid therein is returned to an intermediate pressure stage of the compression device. In an embodiment, the compression device is a pair of compressors disposed in series and the refrigerant vapor is used to cool the refrigerant vapor discharged from the first compressor before it passes into the second compressor.